In recent years, much effort has been pursued in developing solid state electrolytes for Li-ion batteries and beyond. While advances have been made in this field, challenges still exist. A few excellent review papers have been published discussing the mechanism and synthesis of solid state electrolytes with some examples of reducing the interfacial resistances between particles, which provide useful information to understand solid state chemistry underlying the electrolytes. Different from previous overview, this talk will categorize different solid state electrolytes according to their applications including Li-ion batteries, Li-air, Li-S batteries and Li metal protection. Both fundamental science and practical applications will be discussed and the energy density will be projected to compare with state-of-art Li-ion batteries.